linux-kernel - Re: [PATCH v3] blk-cgroup: Replace u64 sync with spinlock for iostat update

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Message-ID: <ZpgMajKn2O521H2s@slm.duckdns.org>
Date: Wed, 17 Jul 2024 08:24:42 -1000
From: "tj@...nel.org" <tj@...nel.org>
To: Waiman Long <longman@...hat.com>
Cc: Boy Wu (吳勃誼) <Boy.Wu@...iatek.com>,
	"boris@....io" <boris@....io>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	Iverlin Wang (王苳霖) <Iverlin.Wang@...iatek.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v3] blk-cgroup: Replace u64 sync with spinlock for iostat
 update

Hello,

On Wed, Jul 17, 2024 at 02:18:39PM -0400, Waiman Long wrote:
> Well, it can be confusing whether we are dealing with blkg->iostat or
> blkg->iostat_cpu. In many cases, we are dealing with iostat_cpu instead of
> iostat like __blkcg_rstat_flush() and blkg_clear_stat(). So we can't
> eliminate the use of u64_stats_update_begin_irqsave() in those cases.

I mean, we need to distinguish them. For 32bits, blkg->iostat has multiple
updaters, so we can't use u64_sync; however, blkg->iostat_cpu has only one
updater (except blkg_clear_stat() which I don't think we need to worry too
much about), so u64_sync is fine.

Thanks.

-- 
tejun